Assombalonga scored his first goal of the season with a curling finishCredit: PA:Empics Sport

Boro hit the woodwork twice and spurned other opportunities, but having been solid in defence and rarely in danger of conceding Tony Pulis will be satisfied with his team's display.

But for Birmingham's former Boro manager Garry Monk it was a day to forget, as it was for Craig Gardner who was red-carded in the closing stages.

Assombalonga scored in the 13th minute, curling the ball home from the left side of the penalty area with his right foot.

It took a slight deflection off visiting right-back Maxime Colin, enough to take it away from goalkeeper Connal Trueman, but there was no doubting the goal will be credited Assombalonga, who had received possession after a superbly-weighted pass by Jonny Howson.

Premier League Round-Up : Watch all the goals from Friday and Saturday's games

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Craig Gardner was sent-off late on for a poor tackle on Jonny HowsonCredit: PA:Empics Sport

In the seconds prior to the strike there had been a brief moment of alarm for Boro, Jacques Maghoma's cross being flicked away by George Friend close to goal, but thereafter Pulis' side looked in control.

Assombalonga had another chance, Michael Morrison flinging himself at the ball to block the resulting shot, and after a set-piece Ryan Shotton and Stewart Downing got in each other's way in the penalty area when a shooting opportunity presented itself.

Boro were better all over the pitch, smothering any attacks that came their way and again went close to going further ahead when Downing struck the inside of the post with a 20-yard free-kick.

The second goal Middlesbrough merited should have come just before the break, but this time Assombalonga hit the post after latching on to Martin Braithwaite's back-heel.

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Tony Pulis' side remain unbeaten this season and are top of the leagueCredit: PA:Empics Sport

Maghoma gets a shot away with Ryan Shotton trying to make the blockCredit: PA:Empics Sport

Boro then missed two more chances in quick succession. Howson created space for himself but saw his shot cleared off the line by Morrison and, with Birmingham unable to clear, Howson headed across goal for Assombalonga, but at a stretch he poked wide.

In in the 82nd minute Gardner was sent off after a challenge on Howson in the middle of Birmingham's half when competing for a loose ball, completing a miserable day for the visitors.